Kinetics study of YBCO thin film epitaxic growth on LAO(100) single crystals by the TFA-MOD method

摘要

We have studied in detail the kinetics aspects of YBa_2Cu_3O_(7-x) (YBCO) thin film growth on LaAlO_3(100) single crystals by the sol-gel TFA-MOD method, using an in situ fluoride selective electrode. Kinetics curves show that an intermediate step exists before the YBCO formation. This step, which starts at a lower temperature than YBCO formation, consists in a partial elimination of F from the BaF_2 precursor to form an oxyfluoride intermediate compound. The total YBCO formation appears at temperatures as high as 700 deg C and the activation energy of the reaction is E_a approx 230 kJ mol~(-1). We have found that at low flow carrier gas rates, the reaction is controlled by diffusion mechanisms and the apparent order of YBCO formation is n = I with respect to the stirring rate, but at higher flow rates the YBCO formation is controlled chemically, then the apparent order is zero. The apparent Ea for the oxyfluoride formation at lower temperatures corresponding to the intermediate step is only approx18 kJ mol~(-1) and the oxyfluoride rate formation increases with the stirring rate, indicating that this intermediate reaction is controlled by diffusion mechanisms.